1. Technical Field
This invention relates generally to image transfer technology, and more specifically to electrophotography. I have invented a laser printer developer which may be used to produce multi-color images with liquid toners.
2. Background Art
In electrophotography, a latent image is created on the surface of an insulating, photo-conducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment components dispersed in an insulating carrier liquid. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photoconductor and toner have the same polarity.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and passed closed to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. Thermal energy may also be used to assist transfer of the image to paper or intermediate transfer medium. A set of fuser rollers melts and fixes the toner in the paper, for the case where no thermal transfer is used, subsequent to direct transfer or indirect transfer when using an intermediate transfer medium, producing the printed image.
There is a demand in the laser printer industry for multi-colored images. Responding to this demand, designers have turned to liquid toners, with pigment components and thermoplastic components dispersed in a liquid carrier medium, usually aliphatic hydrocarbon liquids. With liquid toners, it has been discovered, the basic printing colors--yellow, magenta, cyan and black, may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate medium to produce a multi-colored image.
With liquid toners, however, there is a need to remove the liquid carrier medium from the photoconductor surface after the toner has been applied to it. This way, the photoconductor surface will not transfer the liquid carrier to the paper or to the intermediate medium in the image transfer step(s). Also, this way the liquid carrier may be recovered for recycle and reuse in the developer system, providing economy in terms of printing supplies, and eliminating environmental and health concerns from disposal of excess liquid carrier medium.
It is known from U.S. Pat. No. 3,955,533 to employ a reverse direction roller spaced about 50 microns (about 0.002 inches) from the photoconductor surface to shear off the carrier liquid and excess pigmented solids in the region beyond the outer edge of the image to leave relatively clean background areas on the photoconductor surface.
Also, from U.S. Pat. No. 3,957,016, it is known in a negative toner system to use a positive biased reverse roller maintained at a voltage intermediate the image and background voltages to help clean the background and compact the image on the photoconductor surface.
Also, from U.S. Pat. No. 4,286,039, it is known in a positive toner system to use a reverse roller followed by a negatively biased squeegee roller. The squeegee roller both compacts the latent image and removes excess carrier liquid.
U.S. Pat. Nos. 4,974,027 and 4,999,677 disclose a positively biased reverse roller followed by a negatively biased rigidizing roller followed by a squeegee roller, separate from the rigidizing roller, for removing excess carrier liquid from the image after rigidization. The charge on these rollers may be reversed if the charge on the toner is reversed. In these two patents, an intermediate transfer drum is downstream of the rigidizing roller for receiving the toner image from the photoconductor surface and transferring the image to a sheet of paper.
There is a need in the electrophotography industry then, for a liquid toner developer which provides a rigid latent image leaving the developer unit which is very dry and suitable for direct contact with the paper or intermediate transfer medium onto which the image will be transferred. Also, there is a need for a developer in which the developer roller and rigidizing or squeegee roller are in close proximity to provide a compact developer unit and to minimize any effect of electrostatic charge loss of the toner on the photoconductive surface between the two rollers. Also, there is a need to provide a liquid toner drain path along the length of the rigidizing/squeegee roller to aid excess toner removal from the roller. Also, there is a need for a developer in which the developer roller and rigidizing or squeegee roller are cleaned continuously of residual toner by a common cleaning means which is in contact with both the developer roller and the rigidizing or squeegee roller. Also, there is a need, if the common cleaning means above is a foam roller, for a squeeze rod/roller in contact with the foam roller for removal of toner and carrier liquid from it.